As an optical-path conversion device used for a conventional imaging apparatus, there have been proposed a phase modulation mask, a liquid-crystal varifocal lens, an optical lens, a prism, and the like.
The proposed optical-path conversion device includes (i) a device which varies an optical-path using an optical element having a geometrically unique shape (see Patent Literature (PTL) 1, for example), and (ii) a device which varies an optical-path by applying a voltage on an optical element so as to vary a refraction index (see PTL 2).
In PTL 1, a pair of phase modulation masks are used as the optical-path conversion device. One of the pair of the phase modulation masks is rotated with respect to the other one to cause the optical-path to be varied, thereby enlarge a depth of field of an optical system.
Specifically, each of the pair of the phase modulation masks is a member which has one surface having a plane shape and the other surface having a curved shape expressed by z=kx3 (k is a constant). In addition, the pair of the phase modulation masks are arranged in such a manner that one is fixed and the other one is rotatable in a range within 90 degrees.
As a default position, the rotatable phase modulation mask is arranged so as to cancel phase modulation which occurs in the fixed phase modulation mask. At this time, the pair of the phase modulation masks substantially operates as plane plates parallel to each other, and provides no phase modulation to light passing the pair of the phase modulation masks.
Next, the rotatable phase modulation mask is rotated at 90 degrees. At this time, the pair of the phase modulation masks provide the phase modulation to the light passing therethrough. For example, the light which passes through the pair of the phase modulation masks is subjected to modulation in an X-direction and a Y-direction, so that the optical-path is two-dimensionally varied.
As described above, a relative positional relationship between the paired phase modulation masks is shifted at 90 degrees about an optical axis of an optical lens, so that the pair of the phase modulation masks can produce a modulation function similar to that produced by a cubic phase modulation mask. Here the modulation function is expressed by z=k(x3+y3) (here, k is a constant). In this case, the light which has passed through the pair of the phase modulation masks is inflected and converted to a beneficial light flux, so that the pair of the phase modulation masks can enlarge the depth of field of the optical system.
Accordingly, the pair of the phase modulation masks disclosed in PTL 1 are rotated at 90 degrees, thereby switching states between a state in which the phase modulation is not provided and a state in which the phase modulation is provided, for the light which passes through the pair of the phase modulation masks. This means that the pair of the phase modulation masks can vary the optical-path.
Furthermore, as disclosed in PTL 2, a liquid-crystal varifocal lens is proposed which varies the optical-path using variation in a refractive index of a liquid crystal.
The liquid-crystal varifocal lens is a lens-shaped liquid crystal cell in which liquid crystal molecules are arranged concentrically or radially. An electric field or a magnetic field is applied from an outside to the liquid-crystal varifocal lens to control an arrangement of the liquid-crystal molecules, so that the refractive index of the liquid crystal is continuously varied, thereby varying a focal distance.
As described above, the optical-path can be easily varied by applying a voltage to the liquid-crystal molecules. Therefore, the liquid-crystal varifocal lens is used as the optical-path conversion device.